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The AAPG/Datapages Combined Publications Database

GCAGS Transactions

Abstract


Gulf Coast Association of Geological Societies Transactions
Vol. 41 (1991), Pages 651-666

Surficial Geology of the Houston Area: An Offlapping Series of Pleistocene (& Pliocene?) Highest-Sealevel Fluviodeltaic Sequences

DeWitt C. Van Siclen (1)

ABSTRACT

Most near-surface sediments around Houston were deposited by the Brazos and Trinity rivers during Pleistocene interglacial (high sealevel) stages as a series of offlapping, seaward-thickening, fluviodeltaic sequences. During glacial stages, the landward portion of these sequences was exposed to weathering and erosion before the next high-sealevel strata were deposited. Slow southeast regional tilting elevated the exposed sequences enough that some were never covered entirely by later deposits. These updip margins, termed coastal terraces, now form most of the land surface in the Houston area.

Each coastal terrace south of upper Spring Creek has its own pattern of former natural levees, termed meander-belt ridges (MBRs), which degrade increasingly landward. The MBR pattern on younger terraces truncates those on contiguous older ones and so initially blocked their seaward-flowing drainage. The drainage ponded and broke through the truncated MBRs, forming larger streams that closely follow the sequence boundaries, herein termed gathering streams. Practically all the rest of the present-day drainage follows former back-swamps and flood-basins, leaving the MBRs as divides, even on the youngest terrace. This inversion of the drainage and development of gathering streams makes it possible to reconstruct the MBR patterns from the drainage, even where ridge preservation is poor.

The MBR patterns in a 5,700 mi2 area were mapped from USGS topographic sheets, to produce an improved areal geologic map based on these genetic relationships, i.e., the final sediment transport routes and depositional sequence boundaries. These contrast with traditional mapping criteria which, in the usual absence of outcrops, depend largely on modification by later processes, such as regional tilting and intensity of soil development.

The new map shows that, since deposition of the Pliocene Goliad Formation, eight coastal terraces separated by gathering streams have built eastward from the present Brazos Valley toward the relatively sediment-starved San Jacinto Valley and its Galveston Bay estuary. Farther east, the Trinity River contributed equivalent terraces directly and by occasionally occupying the San Jacinto Valley. It now appears that stratigraphic sequences representing all the Pleistocene highest sealevel stages of Beard, Sangree, and Smith (1982) may be present at the surface in this area, as well as two "extra" ones in the Plio-Pleistocene transition.


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